The invention concerns an assembly for the pressure-tight separation of a first waveguide from a second waveguide, a pressure-resistant conductor body being arranged between the first waveguide and the second waveguide. The invention also concerns a method of producing an assembly of this kind.
An assembly of this kind serves for coupling electromagnetic waves, for example microwaves, which are generated by an electronic circuit, into hermetically separated areas, for example hazardous areas or closed metal containers.
In the prior art, two ways of conducting the electromagnetic waves through a barrier closing off such an area are known in particular. One way of achieving vacuum-tight feeding into a waveguide is to use a metal pin fused into glass as a glass bushing. Glass bushings of this kind are offered in a wide variety of designs, for example by the Schott company. One problem of a glass bushing of this kind is that it is very limited in the extent to which it can be subjected to pressure and temperature. Since the coefficient of thermal expansion of glass and of the metal pin fused in it differ, thermal loading causes high stresses which may lead to damage. This can be compensated only to a limited extent by a shrunk-on metal ring which applies a compressive stress to the insulator formed by the glass. Temperatures in the range from 100 to 200xc2x0 C. are usually permissible; in extreme cases, glass bushings of this kind can be subjected to a maximum of 350xc2x0 C. In this case it must be ensured, however, that a high pressure is not simultaneously applied to the glass bushing. There is a further problem with regard to the permeability with respect to microwaves. Good permeability requires the metal pin to be of a short length. This, however, gives rise to problems with regard to the available soldering length. If, on the other hand, the length of the pin is increased to provide the soldering length required by some approval procedures, the permeability with respect to microwaves decreases greatly.
Another way is to arrange in the waveguide a window which is permeable with respect to electromagnetic waves, in particular microwaves. A window of this kind generally comprises a soldered-in or fused-in glass element in the waveguide. The biggest problem with windows of this kind is the low pressure resistance. Windows of this kind can usually only be exposed to a pressure of a few bars. This is attributable to the fact that the glass elements have only a very small thickness. This small thickness is required, however, for the high bandwidths usually desired.
The object of the invention is to provide a pressure-tight waveguide bushing which has a temperature resistance in the range of 250xc2x0 C. and above as well as a pressure resistance in the range from 60 to 100 bar. At the same time, the bandwidth is to be adaptable to the respective requirements without any great effort.
This object is achieved according to the invention by an assembly for the pressure-tight separation of a first waveguide from a second waveguide, having a pressure-resistant conductor body and a first adaptor, which is arranged between the first waveguide and the end of the conductor body facing the latter, and a second adaptor, which is arranged between the second waveguide and the end of the conductor body facing the latter, the dielectric constants of the first and second adaptors lying between those of the conductor body and the waveguides.
In simplified terms, the assembly according to the invention is based on providing various components for the various functions which have to be performed by a bushing through a waveguide. The pressure resistance is ensured by the conductor body, the material of which can be adapted specifically to requirements. Apart from ceramic, the material glass, in particular quartz glass, is particularly suitable as the material for the conductor body. The adaptors serve to compensate for the transitional locations. Their dielectric constant is chosen such that the optimum transmission behavior is achieved. Plastic, in particular polytetrafluoroethylene, may be used as the material for the adaptor.
According to a preferred embodiment, it is provided that the conductor body has a circular cross section and is surrounded by a metal sheath. With this design, particularly high strength of the assembly is obtained. Furthermore, the metal sheath can be used particularly well for connection to other components, for example by welding.
It is preferably provided that the metal sheath exerts a compressive stress on the conductor body. This design takes into account the fact that, although the materials ceramic and glass, which are preferably used for the conductor body, have a very high compressive strength, they have a very low tensile strength. If a compressive stress is applied to the conductor body by the metal sheath, this compressive stress is superposed on all the stresses which can act on the conductor body during operation. Even if tensile stresses are introduced into the conductor body in the process, the resultant loading is in any event a compressive stress, so that damage to the conductor body is ruled out.
The compressive stress to be exerted by the metal sheath may preferably be generated by the conductor body and the metal sheath being dimensioned in a way appropriate for a press fit. Suitable in particular as the material for the metal sheath which withstands the stresses occurring is an alloy known as Hastelloy or an alloy with the material number 1.4571.
To achieve the desired gas-tight and pressure-resistant separation, customary testing and approval procedures require the conductor body to be of a minimum length of 10 to 15 mm. For microwaves in the K band, this means that the length of the conductor body is a multiple of the diameter. The length of the conductor body can be chosen for a specific frequency (center frequency) such that it is xcex/4 or xcex/4+ a multiple of xcex/2. The reflections at the boundary surfaces opposite one another thus cancel one another out, so that transmission is optimal, in other words reflection attenuation is very great. With a diameter of the conductor body in the range between 4 and 4.5 mm, this gives a length in the range between 8 and 20 mm, preferably between 10 and 15 mm. It should be noted that a change in frequency results in a change in the reflection attenuation if the length of the conductor body remains the same. The longer the conductor body, the smaller the attainable bandwidth.
The adaptors arranged at both ends of the conductor body are preferably captively held in adaptor holders. The adaptors may be formed as stepped cylinders with a first portion and a second portion, the first portion in each case having a greater diameter than the second portion and the first portion in each case facing the conductor body.
An assembly such as that which has been described above may be, in particular, an electrical device with a contact pin and an antenna, the contact pin radiating into the first waveguide and the antenna being connected to the second waveguide, and the assembly being arranged between the first and the second waveguides. In this way, a transmitter can radiate via the contact pin and the antenna into a container, which for example is closed off with a pressure-tight seal.
In the case of a method according to the invention of producing an assembly such as that described above, the conductor body is introduced into a metal sheath in such a way that the metal sheath applies a compressive stress to the conductor body. This method is preferably carried out as follows: firstly, the metal sheath is heated to such a temperature that its inside diameter is greater than the outside diameter of the conductor body. Then the conductor body is inserted into the metal sheath. Finally, the metal sheath is cooled, so that it shrinks onto the conductor body. This method makes it possible to set the compressive stresses exerted by the metal sheath on the conductor body appropriately for the respective conditions, in that the inside diameter of the metal sleeve is produced with a predetermined undersize with respect to the outside diameter of the conductor body. A conductor body made of quartz glass is preferably used for this method.
Advantageous embodiments of the invention emerge from the dependent claims.